1. Field of the Invention
This invention relates to a radiation image information recording and reproducing system, and more particularly to a radiation image information recording and reproducing system in which a grid is employed to suppress influence of scattered rays.
2. Description of the Related Art
There has been put into practice in various fields a radiation image information recording and reproducing system in which a radiation image recorded on a recording medium is read out, thereby obtaining an image signal representing the radiation image, a suitable image processing is carried out on the image signal and a visible image is reproduced on the basis of the processed image signal. For example, in a radiation image recording and reproducing system disclosed in U.S. Pat. Nos. 4,258,264; 4,276,473; 4,315,318; 4,387,428; and Japanese Unexamined Patent Publication No. 56(1981)-11395 and the like, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet, the stimulable phosphor sheet is then exposed to stimulating rays, such as a laser beam, which scan the stimulable phosphor sheet and cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation, the light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal, and a visible radiation image of the object is reproduced on a recording material such as photographic material or on a display device such as a cathode ray tube (CRT) display device on the basis of the electric image signal. The stimulable phosphor sheet is a sheet material bearing thereon a layer of a stimulable phosphor. When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as "a stimulable phosphor".
The system is advantageous in that a radiation image which has a wide latitude and an excellent readability can be obtained.
Further we have proposed a radiation image information recording and reproducing system comprising a conveyor means which conveys a stimulable phosphor sheet along a circulation passage, an image recording section which is disposed on the circulation passage and records a radiation image of an object on the stimulable phosphor sheet, an image read-out section which is disposed on the circulation passage and reads out the radiation image stored on the stimulable phosphor sheet, and an erasing section which is disposed on the circulation passage and releases residual radiation energy remaining on the stimulable phosphor sheet after read-out of the radiation image so that the stimulable phosphor sheet can be repeatedly used. See, for instance, U.S. Pat. No. 4,851,679 and Japanese Unexamined Patent Publication No. 3(1991)-238441.
Further, in such a radiation image information recording and reproducing system, a grid is sometimes provided between the recording medium such as a stimulable phosphor sheet and the object in order to prevent radiation scattered by the object from impinging upon the recording medium when a radiation image of the object is recorded on the recording medium. The grid is in the form of a plate which has a certain thickness and comprises a plurality of strip-like regions which are formed of a material such as aluminum which prevents passage of scattered radiation and of a width substantially equal to that of one picture element and a plurality of strip-like regions which are formed of a material such as carbon which is apt to transmit the radiations and of a width substantially equal to that of one picture element, the two kinds of strip-like regions being arranged alternately.
When a radiation image is recorded by the use of such a grid, radiation scattered by the object in directions deviated from the direction in which the recording radiation is projected onto the object cannot impinge upon the recording medium, whereby the contrast of the radiation image of the object can be improved.
However depending upon the kind of the object, the influence of the scattered rays on the radiation image is less. For example, in the case of an object such as a limb which does not contain a complicated organ and is relatively small in thickness, the influence of the scattered rays on the contrast of the radiation image thereof is less and accordingly it is not necessary to use a grid. In contrast, in the case of an object such as the chest or the abdomen which contains complicated organs, a large amount of radiation is scattered and accordingly a grid must be used.
Thus there has been a demand for a system in which a radiation image can be selectively recorded using or not using a grid.
In the aforesaid conventional radiation image recording and reproducing system, though the grid is removable, the user seldom removes the grid in order to record a radiation image without the grid since the grid is large in both size and weight.
That is, the grid of the radiation image recording and reproducing system is kept on the recording and reproducing system and recording which may be effected without use of the grid is generally effected by another system without grid.